Gas discharge lamp with down conversion phosphor

ABSTRACT

A gas discharge lamp fitted with a gas discharge vessel filled with a gas filling suitable for a gas discharge which emits VUV radiation, with a phosphor coating containing a down-conversion phosphor and with means for igniting and maintaining a gas discharge, in which the down-conversion phosphor has, in a host lattice, a pair of activators of a first lanthanoid ion and a second lanthanoid ion and a sensitizer selected from the group formed by the thallium(I) ion and lead(II) ion, is environmentally friendly and has a high lamp efficiency (lamp. The invention also relates to a down-conversion phosphor comprising, in a host lattice, a pair of activators of a first lanthanoid ion and a second lanthanoid ion and a sensitizer selected from the group formed by the thallium(I) ion and lead(II) ion, wherein the sensitizer occupies a crystallographic site with a coordination number C.N.≧10.

The invention relates to a gas discharge lamp fitted with a gasdischarge vessel filled with a gas suitable for supporting a gasdischarge emitting VUV radiation, with a phosphor coating containing adown conversion phosphor and with means for igniting and maintaining agas discharge.

Conventional fluorescent lamps are mercury gas discharge lamps, thelight emission of which is based on a mercury low-pressure gasdischarge. A mercury low-pressure gas discharge emits radiation mainlyin the near UV with a maximum at approximately 254 nm, which isconverted into visible light by UV-phosphors.

The mercury gas discharge lamp has a refined technology and, with regardto the lamp efficiency ρ_(lamp), can only be matched or exceeded withdifficulty by other lamp technologies. The mercury in the gas fillingis, however, increasingly regarded as an environmentally harmful andtoxic substance, which should be avoided as far as possible in modernmass production because of environmental risks in use, production anddisposal.

Therefore, for some time efforts have been concentrated on thedevelopment of alternative lamp technologies.

One of the mercury-free or low-mercury alternatives to the conventionalmercury gas discharge lamp is the xenon low-pressure gas discharge lamp,which has a gas filling containing mainly xenon. A gas discharge in axenon low-pressure gas discharge lamp emits vacuum ultraviolet radiation(VUV radiation) in contrast to the UV radiation of the mercurydischarge. The VUV radiation is generated by excimers, for example Xe₂*,and is a molecular band radiation with a broad spectrum with a maximumin the range about 172 nm. Using this lamp technology, dischargeefficiencies ρ_(dis) of 65% have been already achieved.

Another advantage of the xenon low-pressure gas discharge lamp is theshort response time of the gas discharge, which makes it useful as asignal lamp for automobiles, as a lamp for copier and fax devices and asa water disinfection lamp.

However, although the xenon low-pressure gas discharge lamp achieves adischarge efficiency ρ_(dis), this which is comparable to that of themercury gas discharge lamp, the lamp efficiency ρ_(lamp), of the xenonlow-pressure gas discharge lamp is still clearly below that of themercury gas discharge lamp.

In principles the ρ_(lamp) efficiency ρ_(lamp) consists of thecomponents discharge efficiency ρ_(dis) phosphor efficiency ρ_(phos),the proportion of the generated visible light which leaves the lampρ_(esc) and the proportion ρ_(vuv) of UV radiation generated by thephosphor:ρ_(lamp)=ρ_(dis)·ρ_(phos)·ρ_(esc)·ρ_(vuv)

A handicap of the conventional xenon low-pressure gas discharge lamp isinherent in the conversion of an energy-rich VUV photon with awavelength of around 172 nm to a comparatively low-energy photon with awavelength in the visible spectrum of 400 nm to 700 nm through thephosphor coating of the lamp. This conversion is ineffective inprinciple. Even if the quantum efficiency of the phosphor is close to100%, by conversion of a VUV photon to a visible photon, on average 65%of the energy is lost due to non-radiative transitions.

Surprisingly, however, it has already been possible to develop VUVphosphors, which achieve a quantum efficiency of more than 100% for theconversion of VUV photons to visible photons. This quantum efficiency isachieved in that a VUV quantum with electron energy of 7.3 eV isconverted to two visible quanta with electron energy of approximately2.5 eV. Such phosphors for xenon low-pressure gas discharge lamps areknown from, for example, Rene T. Wegh, Harry Donker, Koentraad D. Oskam,Andries Meijerink “Visible Quantum Cutting in LiGdF₄:Eu³⁺ throughDown-conversion” Science 283, 663.

In analogy to the multi-photon phosphors already known for some time,which through “up-conversion” generate one short-wave photon from twovisible long-wave photons, these new phosphors, which generate twolong-wave photons from one short-wave photon, are known asdown-conversion phosphors.

However, although the quantum efficiency of the known down-conversionphosphors is high, this does not mean that also the phosphor efficiencyη_(phos) is high too. The phosphor efficiency η_(phos) is not onlydetermined by the quantum efficiency but also by the capability of thephosphor to absorb the VUV radiation to be converted. The absorptivityof the known down-conversion phosphor, however, is very poor. Too muchenergy is lost due to undesirable adsorptions in the lattice and hencethe occupation of the excited states is reduced.

From WO 2002097859 an down-conversion phosphor comprising, in a hostlattice, a pair of activators of a first lanthanoid ion and a secondlanthanoid ion and a sensitizer selected from the group formed by thethallium(I) ion, and lead(II) ion with improved absorptivity is known.

Yet though the phosphors according to WO 2002097859 showed improvedabsorptivity, this prior art phosphor still suffers from poor efficacy.

It is believed that the poor efficacy is caused by a back-transfermechanism that occurs from the activator to the sensitizer and hindersthe quantum cutting process.

It is therefore an object of the present invention to develop a gasdischarge lamp fitted with a gas discharge vessel filled with a gassuitable for a gas discharge which emits VUV-radiation, with a phosphorcoating which contains a down-conversion phosphor and with means forigniting and maintaining a gas discharge, the efficiency of which isimproved.

In accordance with the invention, this object is achieved by a gasdischarge lamp fitted with a gas discharge vessel filled with a gasfilling suitable for supporting a gas discharge emitting VUV radiation,with a phosphor coating containing a down-conversion phosphor, and withmeans for igniting and maintaining a gas discharge, in which thedown-conversion phosphor comprises in a host lattice withcrystallographic sites a pair of activators of a first lanthanoid ionand a second lanthanoid ion and a sensitizer, selected from the groupformed by the thallium(I) ion and lead(II) ion, wherein the sensitizeroccupies a crystallographic site with a coordination number C.N.≧10.

Particularly advantageous effects in relation to the state of the artare obtained by the invention if the first lanthanoid ion is thegadolinium(III) ion and the second lanthanoid ion is selected from theholmium(III) ion and the europium(III) ion.

The main advantage of the phosphors according to the invention is bestdescribed in terms of the energy level diagram of FIG. 3, scheme 1.

For the sensitizer it is necessary to consider the excitationefficiency. The main factors, which influence the efficiency, are theexcitation cross section of the sensitizer, concentration, theexcitation mechanism and the sensitizer lifetime. To maximize theexcitation efficiency, the sensitizer must have a large excitationcross-section and large doping concentration.

The excitation cross section is largely dependent on the excitationmechanism.

In a down-conversion phosphor with the Gd³⁺—Eu³⁺ or Gd³⁺—Ho³⁺ coupleincorporated and Tl⁺ or Pb²⁺ located on a highly coordinatedcrystallographic site of a suitable host lattice Tl⁺ or Pb²⁺ can beexcited with VUV light to the A-, B-, C- or D-band. After non-radiativedecay to the A-band, energy is transferred to the ⁶G_(J)-level of Gd³⁺.Afterwards, the down-conversion process can occur. Altogether, anefficient absorption of VUV light, which is less wavelength dependedcompared to ⁸S_(7/2)-⁶G_(J) transition on Gd³⁺ as in the phosphorsaccording to the state of the art, and also efficient energy transfer tothe ⁶G_(J)-level of Gd³⁺ is realized.

Within the scope of the present invention it is preferred that the hostlattice of the down-conversion phosphor is a fluoride.

In one aspect of the present invention the host lattice of thedown-conversion phosphor is a perovskite.

In another aspect of the present invention the host lattice of thedown-conversion phosphor is an elpasolite.

In one embodiment of the invention it is preferred, that thedown-conversion phosphor comprises the gadolinium(III) ion as the firstlanthanoid ion and as the second lanthanoid ion, the holmium(II) ion anda co-activator selected from the group formed by the terbium(III)ion,ytterbium(III) ion, dysprosium(III) ion, europium(III) ion,samarium(III) ion and manganese(II) ion.

It is preferred that the down-conversion phosphor contains the firstlanthanoid ion in a concentration of 10.0 to 99.98 mol %, the secondlanthanoid ion in a concentration of 0.01 to 30.0 mol % and thesensitizer in a concentration of 0.01 to 30.0 mol %.

It is particularly preferred, that the down-conversion phosphor containsthe sensitizer in a concentration of 0.5 mol %.

It may alternatively be preferred, that the down-conversion phosphorcontains the co-activator in a concentration of 0.01 to 30.0 mol %.

It is particularly preferred, that the down-conversion phosphor containsthe co-activator in a concentration of 0.5 mol %.

The invention also relates to a down-conversion phosphor comprises in ahost lattice with crystallographic sites a pair of activators of a firstlanthanoid ion and a second lanthanoid ion and a sensitizer, selectedfrom the group formed by the thallium(I) ion and lead(II) ion, whereinthe sensitizer occupies a crystallographic site with a coordinationnumber C.N.≧10.

The phosphor is characterized by high quantum efficiency, highabsorption of VUV photons and, in addition, high chemical resistance, sothat said phosphor is particularly suitable for commercial applications,also in plasma display screens. Such a phosphor can also advantageouslybe used for signal lamps in motor vehicles.

The invention is now described in more detail.

FIG. 1 contains information on a state of the art energy transfermechanism based on the Gd³⁺—Eu³⁺ couple.

FIG. 2 contains information on a state of the art energy transfermechanism based on the Gd³⁺—Ho³⁺ couple.

FIG. 3 contains information on the sensitization of Gd³⁺—Eu³⁺ andGd³⁺—Ho³⁺ couple with s²-ions (e.g. Pb²⁺).

A gas discharge lamp according to the invention comprises a gasdischarge vessel with a gas filling and with at least one wall having asurface that is at least partially transparent to visible radiation andthat is provided with a phosphor layer. The phosphor layer contains aphosphor preparation with a down-conversion phosphor of an inorganiccrystalline host lattice, which has obtained its luminous power fromactivation through an activator pair of a first and a second lanthanoidion. The down-conversion phosphor is sensitized by a sensitizer selectedfrom the group formed by the thallium (I) ion and lead (II). Thesensitizer occupies a crystallographic site with a coordination numberC.N.≧10. In addition, the gas discharge lamp is fitted with an electrodestructure to ignite the gas discharge and with further means to igniteand maintain the gas discharge.

Preferably, the gas discharge lamp is a xenon low-pressure gas dischargelamp. Various types of xenon low-pressure gas discharge lamps are knownwhich differ in the ignition of the gas discharge. The spectrum of thegas discharge first contains a high proportion of VUV radiationinvisible to the human eye, which is converted into visible light in thecoating of VUV phosphors on the inside of the gas discharge vessel andthen emitted.

The term “vacuum ultraviolet radiation” below also refers toelectromagnetic radiation with a maximum emission in a wavelength rangebetween 145 and 185 nm.

In a typical construction for the gas discharge lamp, this consists of acylindrical glass lamp bulb filled with xenon, on the wall of which onthe outside is arranged a pair of strip-like electrodes which areelectrically insulated from each other. The strip-like electrodes extendover the entire length of the lamp bulb, where their long sides lieopposite each other leaving two gaps. The electrodes are connected tothe poles of a high voltage source operated with an alternating voltageof the order of 20 kHz to 500 kHz, such that an electric discharge formsonly in the area of the inner surface of the lamp bulb.

When an alternating voltage is applied to the electrodes, in the xenoncontaining filler gas a corona discharge can be ignited. As a result, inthe xenon are formed excimers, i.e. molecules that consist of an excitedxenon atom and a xenon atom in the basic stateXe+X*=Xe₂*.

The excitation energy is emitted again as VUV radiation with awavelength of X=170 to 190 nm. This conversion from electron energy intoUV radiation is highly efficient. The generated VUV photons are absorbedby the phosphors of the phosphor layer and the excitation energy ispartly emitted again in the longer wavelength range of the spectrum.

In principle, for the discharge vessel a multiplicity of forms arepossible, such as plates, single tubes, coaxial tubes, straight,U-shaped, circularly curved or coiled, cylindrical or other shapedischarge tubes.

As a material for the discharge vessel quartz or glass types are used.

The electrodes consist of a metal, for example aluminum or silver, ametal alloy or a transparent conductive inorganic compound, for exampleITO. They can be formed as a coating, an adhesive foil, a wire or a wiremesh.

The discharge vessel is filled with a gas mixture containing an inertgas such as xenon, krypton, neon or helium. Gas fillings, which mainlyconsist of oxygen-free xenon having a low gas pressure, for example 2Torr are preferred. The gas filling can also contain a small quantity ofmercury in order to maintain a low gas pressure during discharge.

The inner wall of the gas discharge vessel is coated partly or fullywith a phosphor coating, which contains one or more phosphors orphosphor preparations. The phosphor layer can also contain organic orinorganic binding agents or a binding agent combination.

The phosphor coating is preferably applied to the inner wall of the gasdischarge vessel as a substrate and can comprise a single phosphor layeror several phosphor layers, in particular double layers of a base and acover layer.

A phosphor coating with a base and a cover layer allows the quantity ofdown-conversion phosphor in the cover layer to be reduced and in thebase layer a less costly phosphor to be used. The base layer preferablycontains as a phosphor a calcium halophosphate phosphor selected so asto achieve the desired shade of the lamp.

The cover layer contains the down-conversion phosphor, which thusconverts an essential part of the VUV radiation generated by the gasdischarge directly into the desired radiation in the visible range.

An important characteristic of the down-conversion phosphor inaccordance with the invention resides in that it comprises a pair ofactivators of a first and a second lanthanoid ion and a sensitizer in ahost lattice, wherein the sensitizer is selected from the group formedby the thallium(I) ion and lead(II) ion, and occupies a crystallographicsite with a coordination number C.N.≧10.

Preferably the first lanthanoid ion is the gadolinium(III) ion and thesecond lanthanoid ion is selected from the holmium(III) ion and theeuropium(III) ion.

In a phosphor according to the present invention any halogen or mixtureof halogens can be used as anions. In a preferred embodiment of theinvention fluorides are used.

Suitable host lattices for the formation of phosphors include a)perovskite related structures, b) elpasolites and c) ternary gadoliniumfluorides of the MGd₂F₇-type.

a) The general formula of the perovskite related structure, usefulaccording to the present invention, is M′M″GdF6 with M′=Li, Na, K, Rb,Cs, Cu, Ag and M″=Be, Mg, Ca, Sr, Ba, Zn.

The chemical constitution of the ideal perovskite structure can berepresented by the general formula ABX₃. The perovskite structure isbuilt up of cubes consisting of three chemical elements A, B and X in aratio of 1:1:3, respectively. The A and B atoms are incorporated ascations, the X atoms, usually fluorine, as anions. The size of the Acation is always comparable to that of fluorine, whereas the B cation ismuch smaller. The valency of the individual cations may vary, providedthat the sum of the cation valencies counterbalances the charge of thethree anions.

In the ideal, undistorted perovskite structure the anions and the Acations form a cubical closest packing, so that the A site is surroundedby 12 anions and the coordination number C.N. equals 12.

The B cations occupy the octahedral vacancies in the lattice that areformed only by 6 anions.

Variations in the composition of the perovskites lead to the formationof more or less distorted, perovskite structures whose symmetry is lesshigh.

Variations of the compounds having the perovskite structure are formedin that the A and/or B cations can be partly substituted by one or moreother cations, so that the initially ternary perovskites ABX₃ are turnedinto perovskites having more elements, for example quaternary, quinary,senary, septenary, etc. perovskites.

Examples of cations that may substitute gadolinium on an B site areCe³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺ with aconcentration of 0.01 to 30 mol-%,

Al³⁺, Ga³⁺, In³⁺, Sc³⁺, Y³⁺, La³⁺ with a concentration of 0.01 to 90mol-%.

The perovskite related structure of M′M″GdF₆ having cation vacancies onthe B sites with large resulting anisotropic environments arecharacterized by large crystal field splittings which significantlyimprove the absorption of VUV-radiation by the ion pairs Gd³⁺—Eu³⁺ andGd³⁺—Ho³⁺. Large crystal field splittings also result in increasedopportunity for internal relaxation mechanisms involving photongeneration, which thus far have not been found to be pronounced incomparable but more isotropic media.

b) The general formula of the elpasolites, useful according to thepresent invention is:

A_(2−y)B_(1+y)Me³⁺X₆, wherein A is a monovalent ion such as Li, Na, K,Rb, Cs, Cu, Ag, B is a monovalent ion such as Li, Na, K, Rb, Cs, Cu, Ag,A is different from B, Me³⁺ is a trivalent ion, preferably gadolinium, Xis at least one of F, Cl, Br and I, 0<y<1 and 0<x<0.3.

Examples of cations that may substitute gadolinium on an B site areCe³⁺, Pr⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺ with aconcentration of 0.01 to 30 mol-%,

Al³⁺, Ga³⁺, In³⁺, Sc³⁺, Y³⁺, La³⁺ with a concentration of 0.01 to 90mol-%.

The crystallography of elpasolites is related to the better-knownperovskites.

Elpasolites can, depending on the ionic radii of the various ionscomposing the compound, crystallize in various crystalline systems.Cubic, triclinic and hexagonal elpasolites are known. Elpasolitescrystallized in any crystalline system are useful for the presentinvention.

c) Ternary gadolinium fluorides MGd₂F₇. comprise a host lattice whereinthe C.N. of the M-cation is 14. Twelve anions are arranged in a firstcoordination sphere, two additional anions are arranged in a secondcoordination sphere.

Due to the high coordination numbers and the non-polar ligands, thesehost lattices are characterized by a low ligand field for cations, whichare part of the host lattice.

While the structural considerations are paramount, the compositions mustalso contain the requisite ion pairs Gd³⁺—Eu³⁺, Gd³⁺—Ho³⁺ and mixturesthereof. Gadolinium is partly exchanged in the host lattice by Eu³⁺ witha concentration of 0.01 to 30 mol-% or by Ho³⁺ with a concentration of0.01 to 30 mol-%,

The phosphors doped with the activator pair Gd³⁺—Eu³⁺ or Gd³⁺—Ho³⁺preferably contain 10 to 99.8 mol % of the trivalent Gd³⁺ and 0.01 to 30mol %, particularly preferably 1.0 mol % of the trivalent holmium or thetrivalent europium.

The pair of activators of a first lanthanoid ion and a second lanthanoidion and the co-activator ion cooperate in the sequential emission ofphotons by means of which the phosphor generates more than one visiblephoton from a UV photon.

Another requirement is the incorporation of a sensitizer in the hostlattice on a highly coordinated crystallographic site. The sensitizeratoms absorb the incident photon either directly, or from the hostlattice, and transfer it to the activator ion.

The sensitizer is selected from the group formed by the thallium(I) ionand lead(II) ion. In general, these ions are also indicated inaccordance with their electron configuration as 6s2 ions.

Tl⁺ or Pb²⁺ are incorporated in the fluoride host lattice on a highlycoordinated crystallographic site, preferably on the M′ or M″ site of aperovskite-related type of structure with a composition M′M″GdF₆ andwith M′=Li, Na, K, Rb, Cs, Cu, Ag and M″=Be, Mg, Ca, Sr, Ba, Zn and Tl⁺or Pb²⁺ coordinated by 12 fluoride ions, preferably on the M′ site of anelpasolite type of structure with a composition M′₂M″GdF₆ and withM′=Li, Na, K, Rb, Cs, Cu, Ag and M″=Li, Na, K, Rb, Cs, Cu, Ag and Tl⁺ orPb²⁺ coordinated by 12 fluoride ions, preferably on the M site of astructure with a composition MGd₂F₇ and with M=Be, Mg, Ca, Sr, Ba, Znand Tl⁺ or Pb²⁺ coordinated by 14 fluoride ions;

The sensitizer enhances the sensitivity of the down-conversion phosphorto VUV radiation and makes it less wavelength-dependent. The sensitizerhas a high intrinsic absorption in the desired VUV range of 100 to 200nm, which exceeds the intrinsic absorption of the non-sensitizeddown-conversion phosphors at approximately 183, 195 and 202 nm. Thetransmission of the excitation energy to the pair of activators issubject to losses because lattice imperfections cause excitation statestraversing the lattice to release energy to said lattice in the form ofheat oscillations. Next, the reduced, absorbed excitation energy istransferred to the activator and triggers the down-conversion mechanism.This leads to increased luminescence of the down-conversion phosphor asit has been “sensitized” by the sensitizer so as to be able to beluminescent upon exposure to VUV radiation.

The down-conversion phosphor may additionally also comprise aco-activator.

The co-activator is selected from the group of the trivalent ions ofterbium, ytterbium, dysprosium and samarium and the bivalent ions ofmanganese.

The current inventors believe the following possible mechanism of energytransfer utilizing Gd³⁺—Eu³⁺ or Gd³⁺—Ho³⁺ ion pairs together with s²ions, such as Tl(I) or Pb(II) as sensitizers.

The Tl(I) or Pb(II) sensitizers absorb the incident VUV radiation (λbetween 100 and 200 nm) and transfer the energy to the Gd³⁺⁶G_(J) states(or to a level higher in energy than ⁶G_(J)) (FIG. 3).

In case of the Gd³⁺—Eu³⁺ ion pair the excitation mechanism can takeplace by a Gd³⁺⁸S_(7/2)-⁶G_(J) excitation or excitation to a levelhigher in energy than ⁶G_(J) of the gadolinium(III) ion, after which across-relaxation transition Gd³⁺⁶G_(J)-⁶P_(J)/Eu³⁺⁷F₁-⁵D₀ between thegadolinium(III) ion and the europium(III) ion takes place.

Next the europium(III) ion emits a first photon in the visible, theenergy of which corresponds to the transition Eu³⁺⁵D₀-⁷F_(J).

The gadolinium(III) ion then transfers the energy to anothereuropium(III) ion in the sublattice by aGd³⁺⁶P_(J)—⁸S_(7/2)/Eu³⁺⁷F₁-⁵D_(J) transfer and a Eu³⁺⁵D_(J)-⁷F_(J)emission generates a second photon in the visible.

In case of Gd³⁺—Ho³⁺ ion pair the excitation mechanism can take place bya Gd³⁺⁸S_(7/2)-⁶G_(J) excitation or excitation to a level higher inenergy than ⁶G_(J) of the gadolinium(III) ion, after which across-relaxation transition Gd³⁺⁶G_(J)-⁶P_(J)/Ho³⁺⁵I₈—⁵F₅ between thegadolinium(III) ion and the holmium(III) ion takes place.

Next the holmium(III) ion emits a first photon in the visible, theenergy of which corresponds to the transition Ho³⁺⁵F₅—⁵I₈.

The gadolinium(III) ion then transfers the energy to anothereuropium(III) ion in the sublattice by aGd³⁺⁶P_(J)—⁸S_(7/2)/Eu³⁺⁷F₁-⁵D_(J) transfer and a Eu³⁺⁵D_(J)-⁷F_(J)emission generates a second photon in the visible.

After energy transfer of the Gd³⁺⁶P_(J)—⁸S_(7/2) state of thegadolinium(III) ion to a co activator, the emission of the coactivatorgenerating a second photon in the visible.

The emission of two photons in the visible per absorbed VUV photonleading to a down-conversion efficiency between 100 and 200%.

This quantum cutter concept is an improvement with regard to a state ofthe art quantum cutter concept based on interacting rare-earth ions,namely Gd³⁺—Eu³⁺ (FIG. 1) and Gd³⁺—Ho³⁺ (FIG. 2). Typical compounds withthe ion couple incorporated are, for instance, LiGdF₄:Eu or LiGdF₄:Ho,Tb. For these state of the art materials down-conversion efficiencies upto 200% are experimentally proven. However, in view of a technicalapplication up to now the materials suffer from the fact that on onehand their absorption in the VUV is quite low. As a result most of theincoming light is reflected. Moreover, due to the special energytransfer mechanism starting with ⁸S_(7/2)-⁶G_(J) transition on Gd³⁺, theabsorption is limited to three narrow lines at 183, 195 and 202 nm. Allof these lines do not correlate with the maximum of the emission band ofthe Xe-discharge at 172 nm. In accordance with this situation nodown-conversion occurs at 160 to 180 nm excitation. At 202 nm excitationtill now quantum efficiency only reaches about 70% and light outputabout 30%. It should be mentioned explicitly that this is with thedown-conversion effect included.

The absorption coefficients of the sensitized down-conversion phosphorsin accordance with the invention are particularly large for thewavelengths in the range of xenon radiation, and the quantum efficiencylevels are high. The host lattice is not involved in the luminescenceprocess but influences the precise position of the energy levels of theactivator ions and the sensitizer ions and consequently the wavelengthsof absorption and emission.

The emission bands lie in the range from near UV to yellow-orange, butpredominantly in the red and green range of the electromagneticspectrum. The extinction temperature of these phosphors is above 100° C.

The grain size of the phosphor particles is not critical. Normally, thephosphors are used as fine grain powders with a grain-size distributionbetween 1 and 20 um.

As production processes for a phosphor layer on a wall of the dischargevessel, both dry coating processes, for example electrostatic depositionor electrostatically supported sputtering, and wet coating processes,for example dip coating or spraying, can be considered. For wet coatingprocesses, the phosphor preparation must be dispersed in water, anorganic solvent, where applicable together with a dispersion agent, atenside and an anti-foaming agent or a binding agent preparation.Suitable binding agent preparations for a gas discharge lamp accordingto the invention are organic or inorganic binding agents, which arecapable of withstanding an operating temperature of 250° C. withoutdestruction, embrittlement or discoloration.

For example, the phosphor preparation can be applied to a wall of thedischarge vessel by means of a flow coating process. The coatingsuspensions for the flow coating process contain water or an organiccompound such as butyl acetate as the solvent. The suspension isstabilized and its rheological properties influenced by the addition of;auxiliary agents such as stabilizers, liquefiers, cellulose derivatives.The phosphor suspension I is applied to the vessel walls as a thinlayer, dried and burnt in at 600° C.

It can also be preferred that the phosphor preparation for the phosphorlayer is deposited electrostatically on the inside of the dischargevessel.

For a gas discharge lamp which is to emit white light, preferably a blueemitting phosphor from the group BaMgAl₁₀O₁₇:Eu²⁺ and Sr₅(PO₄)₃Cl:Eu²⁺is combined with a red-emitting phosphor from the group RbGd₂F₇:Eu, Tl;KMgF₃:Gd, Eu, Pb; BaGd₂F₇:Eu, Pb; KGd₂F₇:Eu, Bi and with agreen-emitting phosphor from the group (Y, Gd)BO₃:Tb and LaPO₄:Ce, Tb orwith a green-red phosphor such as LiGdF₄:Ho, Tb, Tl. The phosphor layerusually has a layer thickness of 5 to 100 μm.

The vessel is then evacuated to remove all gaseous contaminants, inparticular oxygen. The vessel is then filled with xenon and sealed.

EXAMPLE 1

A cylindrical discharge vessel of glass having a length of 590 mm, adiameter of 24 mm and a wall thickness of 0.8 mm is filled with xenon ata pressure of 200 hPa. The discharge vessel contains an axis-parallelinner electrode in the form of a noble metal rod of 2.2 mm diameter. Onthe outside of the discharge vessel is the outer electrode composed oftwo strips of conductive silver 2 mm in width arranged axis-parallel andconductively connected to the power supply. The lamp is operated bymeans of a pulsed DC voltage.

The inner wall of the discharge vessel is coated with a phosphor layer.

The phosphor layer contains a three-band phosphor mixture with thefollowing components: BaMgAl₁₀O₁₇:Eu²⁺ as the blue component, LaPO₄:Ce,Tb as the green component and KSrdF₆:Eu, Tl as the red component.

To produce KSrGdF₆:Eu, Tl with 1.0 mol % europium and 0.1 mol %thallium, 49.50 g GdF₃, 13.55 g KF, 29.44 g SrF₂, 0.49 g EuF₃ and 0.52 gTlF are thoroughly mixed and ground in an agate mortar. The mixture isprefired in a corundum crucible in a quartz tube under vacuum at apressure of 8 10⁻² Pa for 2 hours at 300° C. During firing, the quartztube was rinsed with argon three times and evacuated again to 8 10⁻² Pa.The oven temperature was then increased at a rate of 5.5° C./min to 750°C. and the mixture sintered for 8 hours at 750° C. The sintered powderwas reground and sieved to a grain size 40 μm. The crystal structure ofthe formed phase was checked by means of X-ray diffractometry.

In this manner, a light output of initially 37 lm/W was achieved. After1000 h operating hours, the light output was approximately 34 lmW. Thequantum efficiency for VUV-light is approximately 70%.

EXAMPLE 2

A cylindrical discharge vessel of glass having a length of 590 mm, adiameter of 24 mm and a wall thickness of 0.8 mm is filled with xenon ata pressure of 200 hPa. The discharge vessel contains an axis-parallelinner electrode in the form of a noble metal rod of 2.2 mm diameter. Onthe outside of the discharge vessel is the outer electrode composed oftwo strips of conductive silver 2 mm in width arranged axis-parallel andconductively connected to the power supply. The lamp is operated bymeans of a pulsed DC voltage.

The inner wall of the discharge vessel is coated with a phosphor layer.

The phosphor layer contains a three-band phosphor mixture with thefollowing components: BaMgAl₁₀O₁₇:Eu²⁺ as the blue component andCsBaGdF₆:Ho, Tb, Pb with Ho(1.0 mol-%), Tb(1.0 mol-%), Pb(1.0 mol-%) asthe green-red component.

To produce CsBaGdF₆:Ho, Tb, Pb with 1.0 mol-% holmium, 1.0 mol-% terbiumand 1.0 mol-% lead, a quantity of 49.00 g GdF₃, 35.51 g CsF, 40.89 gBaF₂, 0.52 g HoF₃, 0.50 g TbF₃ and 0.57 g PbF₂ are thoroughly mixed andground in an agate mortar. The mixture is prefired in a corundumcrucible in a quartz tube under vacuum at a pressure of 8 10⁻² Pa for 2hours at 300° C. During firing, the quartz tube was rinsed with argonthree times and evacuated again to 8 10⁻² Pa. The oven temperature wasthen increased at a rate of 5.5° C./min to 750° C. and the mixturesintered for 8 hours at 750° C. The sintered powder was reground andsieved to a grain size 40 μm. The crystal structure of the formed phasewas checked by means of X-ray diffractometry.

In this manner, a light output of initially 37 lm/W was achieved. After1000 h operating hours, the light output was approximately 34 lm/W. Thequantum efficiency for VUV-light is approximately 70%.

EXAMPLE 3

A cylindrical discharge vessel of glass having a length of 590 mm, adiameter of 24 mm and a wall thickness of 0.8 mm is filled with xenon ata pressure of 200 hPa. The discharge vessel contains an axis-parallelinner electrode in the form of a noble metal rod of 2.2 mm diameter. Onthe outside of the discharge vessel is the outer electrode composed oftwo strips of conductive silver 2 mm in width arranged axis-parallel andconductively connected to the power supply. The lamp is operated bymeans of a pulsed DC voltage.

The inner wall of the discharge vessel is coated with a phosphor layer.

The phosphor layer contains a three-band phosphor mixture with thefollowing components: BaMgAl₁₀O₁₇:Eu²⁺ as the blue component, LaPO₄:Ce,Tb as the green component and Rb₂NaGdF₆: Eu, Pb with 1.0 mol-% europiumand 1.0 mol-% lead as the red component.

To produce Rb₂NaGdF₆: Eu, Pb with 1.0 mol-% europium and 1.0 mol-% lead,a quantity of 49.50 g GdF³, 48.60 g RbF, 9.81 g NaF, 0.49 g EuF₃ and0.57 g PbF₂ are thoroughly mixed and ground in an agate mortar. Themixture is prefired in a corundum crucible in a quartz tube under vacuumat a pressure of 8 10⁻² Pa for 2 hours at 300° C. During firing, thequartz tube was rinsed with argon three times and evacuated again to 810⁻² Pa. The oven temperature was then increased at a rate of 5.5°C./min to 750° C. and the mixture sintered for 8 hours at 750° C. Thesintered powder was reground and sieved to a grain size 40 μm. Thecrystal structure of the formed phase was checked by means of X-raydiffractometry.

In this manner, a light output of initially 37 lm/W was achieved. After1000 h operating hours, the light output was approximately 34 lm/W. Thequantum efficiency for VUV-light is approximately 70%.

EXAMPLE 4

A cylindrical discharge vessel of glass having a length of 590 mm, adiameter of 24 mm and a wall thickness of 0.8 mm is filled with xenon ata pressure of 200 hPa. The discharge vessel contains an axis-parallelinner electrode in the form of a noble metal rod of 2.2 mm diameter. Onthe outside of the discharge vessel is the outer electrode composed oftwo strips of conductive silver 2 nm in width arranged axis-parallel andconductively connected to the power supply. The lamp is operated bymeans of a pulsed DC voltage.

The inner wall of the discharge vessel is coated with a phosphor layer.

The phosphor layer contains a three-band phosphor mixture with thefollowing components: BaMgAl₁₀O₁₇:Eu²⁺ as the blue component, LaPO₄:Ce,Tb as the green component and BaGd₂F₈:Eu, Pb with 1.0 mol-% europium and1.0 mol-% lead as the red component.

To produce BaGd₂F₈:Eu, Pb with 1.0 mol-% europium and 1.0 mol-% lead, aquantity of 49.50 g GdF₃, 20.44 g BaF₂, 0.49 g EuF₃ and 0.28 g PbF₂ arethoroughly mixed and ground in an agate mortar. The mixture is prefiredin a corundum crucible in a quartz tube under vacuum at a pressure of 810⁻² Pa for 2 hours at 300° C. During firing, the quartz tube was rinsedwith argon three times and evacuated again to 8 10⁻² Pa. The oventemperature was then increased at a rate of 5.5° C./min to 750° C. andthe mixture sintered for 8 hours at 750° C. The sintered powder wasreground and sieved to a grain size 40 μm. The crystal structure of theformed phase was checked by means of X-ray diffractometry.

In this manner, a light output of initially 37 lm/W was achieved. After1000 h operating hours, the light output was approximately 34 lm/W. Thequantum efficiency for VUV-light is approximately 70%.

EXAMPLE 5

A cylindrical discharge vessel of glass having a length of 590 mm, adiameter of 24 mm and a wall thickness of 0.8 mm is filled with xenon ata pressure of 200 hPa. The discharge vessel contains an axis-parallelinner electrode in the form of a noble metal rod of 2.2 mm diameter. Onthe outside of the discharge vessel is the outer electrode composed oftwo strips of conductive silver 2 mm in width arranged axis-parallel andconductively connected to the power supply. The lamp is operated bymeans of a pulsed DC voltage.

The inner wall of the discharge vessel is coated with a phosphor layer.

The phosphor layer contains a three-band phosphor mixture with thefollowing components: BaMgAl₁₀O₁₇:Eu²⁺ as the blue component, LaPO₄:Ce,Tb as the green component and Cs₂KGdF₆:Eu, Tl with 1.0 mol-% europiumand 1.0 mol-% thallium as the red component.

To produce Cs₂KGdF₆:Eu, Tl with 1.0 mol-% europium and 1.0 mol-%thallium a quantity of 49.50 g GdF₃, 71.03 g CsF, 13.55 g KF, 0.49 gEuF₃ and 0.52 g TlF are thoroughly mixed and ground in an agate mortar.The mixture is prefired in a corundum crucible in a quartz tube undervacuum at a pressure of 8 10⁻² Pa for 2 hours at 300° C. During firing,the quartz tube was rinsed with argon three times and evacuated again to8 10⁻² Pa. The oven temperature was then increased at a rate of 5.5°C./min to 750° C. and the mixture sintered for 8 hours at 750° C. Thesintered powder was reground and sieved to a grain size 40 μm. Thecrystal structure of the formed phase was checked by means of X-raydiffractometry.

In this manner, a light output of initially 37 lm/W was achieved. After1000 h operating hours, the light output was approximately 34 lm/W. Thequantum efficiency for VUV-light is approximately 70%.

1. A gas discharge lamp fitted with a gas discharge vessel filled with agas filling suitable for supporting a gas discharge emitting VUVradiation, with a phosphor coating containing a down-conversionphosphor, and with means for igniting and maintaining a gas discharge,in which the down-conversion phosphor comprises in a host lattice withcrystallographic sites a pair of activators of a first lanthanoid ionand a second lanthanoid ion and a sensitizer, selected from the groupformed by the thallium(I) ion and lead(II) ion, wherein the sensitizeroccupies a crystallographic site with a coordination number C.N.≧10. 2.A gas discharge lamp as claimed in claim 1, characterized in that thefirst lanthanoid ion is the gadolinium(III) ion and the secondlanthanoid ion is selected from the holmium(III) ion and theeuropium(III) ion.
 3. A gas discharge lamp as claimed in claim 1,characterized in that the host lattice of the down-conversion phosphoris a fluoride.
 4. A gas discharge lamp as claimed in claim 1,characterized in that the host lattice of the down-conversion phosphoris a perovskite.
 5. A gas discharge lamp as claimed in claim 1,characterized in that the host lattice of the down-conversion phosphoris an elpasolite.
 6. A gas discharge lamp as claimed in claim 1,characterized in that the down-conversion phosphor comprises thegadolinium(III) ion as the first lanthanoid ion and, as the secondlanthanoid ion, the holmium(III) ion or the europium(III) ion and aco-activator selected from the group formed by the terbium(III) ion,ytterbium(III) ion, dysprosium(III) ion, europium(III) ion, samarium(II)ion and manganese(II) ion.
 7. A gas discharge lamp as claimed in claim1, characterized in that the down-conversion phosphor contains the firstlanthanoid ion in a concentration of 10.0 to 99.98 mol %, the secondlanthanoid ion in a concentration of 0.01 to 30.0 mol % and thesensitizer in a concentration of 0.01 to 30.0 mol %.
 8. A gas dischargelamp as claimed in claim 1, characterized in that the down-conversionphosphor contains the sensitizer in a concentration of 0.5 mol %.
 9. Agas discharge lamp as claimed in claim 1, characterized in that thedown-conversion phosphor contains the co-activator in a concentration of0.01 to 30.0 mol %.
 10. A down-conversion phosphor which contains, in ahost lattice with crystallographic sites, a pair of activators of afirst lanthanoid ion and a second lanthanoid ion, and a sensitizerselected from the group formed by the thallium(I) ion and lead(II) ion,wherein the sensitizer occupies a crystallographic site with acoordination number C.N.≧10.